Many integrated optical devices specify a precisely controlled separation between parallel waveguides. The required separation may be in the neighborhood of a few tenths of a micron, with the separation required to be constant over a distance of several centimeters. Conventional fabrication techniques which orient waveguides in a side-by-side arrangement on the surface of a substrate do not lend themselves to the achievement of the required accuracy and consistency. This is especially the case when waveguides are made from reflowed glasses.
It is known that certain acousto-optic interactions exhibit increased efficiency if the optical waveguides are arranged vertically (i.e. "stacked") as opposed to a side-by-side arrangement. However, in order to achieve such increased efficiency, a multiple-channel, stacked, optical waveguide structure must have planar surfaces to enable proper channel-to-channel interaction. Thus, it is necessary to maintain a planar surface after each fabrication step.
The prior art teaches a variety of techniques for the fabrication of optical waveguides. In U.S. Pat. No. 3,865,646 to Logan et al., a single or double heterostructure is fabricated from gallium arsenide-aluminum gallium arsenide layers. Liquid phase or molecular beam epitaxy are employed, to superimpose layers, one on the other. Two alternative techniques are employed to construct a waveguide layer. In one, an aluminum gallium arsenide layer is epitaxially grown over a mesa to form a two dimensional waveguide. In the second, edges of an active region of an aluminum gallium arsenide double heterostructure are differentially
to provide a defined waveguide.
U.S. Pat. No. 4,070,516 to Kaiser describes a ceramic module body with incorporated glass channels that enable communication with a semiconductor chip mounted on the body. The process employs ceramic green sheets with incorporated glass paste channels.
U.S. Pat. No. 4,715,672 to Duguay et al. describes a planar silicon dioxide waveguide that is bounded by thin polysilicon, high index layers to provide anti-resonant reflecting surfaces.
U.S. Pat. No. 4,929,302 to Valette describes a procedure for producing optical waveguides wherein an additive process produces juxtaposed optical waveguides. A pair of guide structures are separated by a layer whose refractive index is intermediate the two optical waveguides. U.S. Pat. No. 4,933,262 to Beguin describes a method and structure for interconnecting an optical fiber with a planar optical guide.
In U.S. Pat. No. 4,973,119 to Taki, an optical isolator is described that employs a planar waveguide and a magnetic thin film having a magneto-optic effect. The substrate has a refractive index close to the refractive index of the magnetic thin film and the film is magnetized in a direction lying in a plane substantially normal to the direction in which light is propagated through the waveguide.
U.S. Pat. No. 5,013,129 to Harada et al describes an optical frequency converter wherein an embedded waveguide is surrounding by a cladding which fully reflects the fundamental optical frequency being transmitted, but not its harmonics. U.S. Pat. No. 5,018,809 to Shin et al., describes a planar optical waveguide with a self aligning cladding.
U.S. Pat. No. 5,026,135 to Booth describes a method for producing planar optical waveguides with a glassy coating of doped silicon dioxide that provides a low oxygen transmission value --to prevent waveguide deterioration.
Accordingly, it is an object of this invention to provide an improved method for producing stacked optical waveguides.
It is another object of this invention to provide a method for producing vertically stacked optical waveguides which lends itself to the use of differing optical waveguide materials.
It is still another object of this invention to provide an improved method for producing stacked optical waveguides wherein a high degree of positional precision is obtained.